1. Field of the Invention
This invention relates to carriers for electrophotographic development.
2. Description Of The Prior Art
A number of carriers for electrophotographic development are known including reduced iron powders, atomized iron powders, iron powders obtained by pulverizing iron wastes such as from cutting, and the like. However, this type of carrier is so low in resistance that bias voltage is apt to leak, and black lines are formed on copies obtained by electrophotography, thus making it difficult to read characters or letters in portions where the black lines are superposed thereon.
Another type of carrier is known in which iron powder is oxidized or coated with resins thereby producing a carrier of high electric resistance. However, this type of carrier deteriorates as used over a long time, so that a so-called "excessively edged" image is reproduced by electrophotography in which a solid portion comes off at the center thereof, accompanied by occurrence of fogging. Moreover, in the former case, there is a limitation on the level of resistance imparted to the iron powder. Where a duplicating machine receives defects scratches on the photosensitive drum thereof, bias voltage will leak. This is true of the latter case where the resin coating is thin. Although there is experienced little leakage of bias voltage when the resin coating is thick in the latter case, such a carrier becomes so high in resistance that the resulting images are excessively fringed or edged. With resin-coated carriers, the charging characteristics are determined depending on the type of resin being coated and thus they are used only in combination with specific types of toners.
Although ferrite (MO.Fe.sub.2 O.sub.3) carriers are also known, this type of carrier has such a high resistance that an excessive edging effect is unfavorably produced on images.
Some known carriers described above which have residual magnetism are disadvantageous in that carrier particles attract one another, thus preventing the smooth flow of a developing agent. In apparatus in which the flow of developing agent gives a great influence on a toner concentration control device, the degree of residual magnetism will produce a serious problem.